This application relates to and incorporates herein by reference Japanese patent application no. 2001-27182 filed on Feb. 2, 2001.
The present invention relates to a vehicle driving control device for controlling the driving conditions of a vehicle by control of the engine, or control of both the engine and a braking device, independently of the controls performed by the driver.
A conventional control device performs so-called adaptive cruise control (hereafter, simply ACC), an example of which is disclosed in unexamined Japanese application (JP-A) No.7-47862. The conventional control device controls a trailing vehicle to follow a leading vehicle while keeping a safe distance between the vehicles. The control device computes a first target vehicle speed, which is the speed that is required to follow the leading vehicle. Then a target engine torque, which is the torque necessary to maintain the target vehicle speed, is computed for generating a torque command representing the target engine torque for directing the driving speed of the vehicle to the target vehicle speed.
To compute the target engine torque from the target vehicle speed according to the above-described control device, the basic torque of the engine is computed based on the current running resistance of the vehicle (rolling resistance, air resistance, acceleration resistance, and hill-climbing resistance, which is based on vehicle weight and road gradient), the gear ratio of the power train (the gear ratio of the transmission and differential gears), and the torque ratio of a torque converter. Furthermore, a correction of engine torque (correction torque) is computed from the deviation between the target vehicle speed and the actual vehicle speed. Then, the basic torque is corrected by the correction torque.
In the case of the conventional control device, therefore, the actual torque transmission characteristics of the torque converter will not be reflected in the target engine torque, which is the final control target. Also, it is impossible to set an optimum target engine torque for achieving the target vehicle speed during a transition period during which the engine speed is being greatly changed by the engine torque control and there is significant slippage in the torque converter.
The conventional control device uses an instantaneous value of the torque ratio to represent the characteristics of the torque converter when computing the basic torque of the engine. When the torque converter is locked by a lockup device and the engine is running at a steady speed or when the torque converter is in a relatively stable state because of low slippage, a nearly proper basic torque is obtained. However, during a transition period when the engine speed substantially varies or when the torque converter slips significantly, the characteristics of the torque converter cannot be reflected in the basic torque requirement, and it is impossible to optimally set the target engine torque.
Furthermore, in the conventional control device, the target vehicle speed is determined as the target of the ACC control, and the engine torque is set based on the target vehicle speed in performing engine control. Therefore, if an optimum target engine torque for achieving the target vehicle speed could be set, the acceleration of the vehicle resulting from the engine control would disturb the driver and other passengers. That is, according to the laws of motion, torque and acceleration (and deceleration) are proportional, and the target engine torque and the acceleration (and deceleration) of the vehicle correlate. However, in the conventional control device, since the target engine torque is based on the target vehicle speed, which is an integral value of the acceleration (and deceleration) of the vehicle, it is impossible to set a target engine torque that results in comfortable levels of acceleration. For example, when the target vehicle speed is increased in response to acceleration of the leading vehicle, the trailing vehicle will be abruptly accelerated during a transition period until the target vehicle speed is reached, which creates discomfort for the vehicle occupants. Since it takes time to accelerate the vehicle, the behavior of the vehicle will disturb the occupants.
In view of the above-described problems inherent in known control devices, it is an object of this invention to provide a driving control device that is capable of accelerating and decelerating a vehicle while giving the vehicle occupants a feeling of smooth operation and to optimize the vehicle driving torque of the engine even during a transition period when the engine speed greatly varies and when the slippage of the torque converter is increasing.
To accomplish this object, a target tire operating torque computing means computes the target tire operating torque necessary to operate the vehicle at a predetermined running state on the basis of an input from a sensor that detects the running condition of the vehicle. Then, a driving resistance estimating means estimates the driving resistance of the vehicle on the basis of an input from the sensor.
Then, target turbine torque computing means computes the target turbine torque of the torque converter on the basis of the target tire operating torque, the computed driving resistance, and the gear ratio of the power transmission train. A target engine operating condition computing means computes the target engine torque and the target engine speed by a rule of control set in accordance with the state of the lockup clutch on the basis of the computed turbine torque and the turbine speed of the torque converter.
The engine control means controls the engine in accordance with the result of a computation by the target engine control target computing means so that the engine torque and the engine speed will be directed to the target engine torque and the target engine speed.
In the driving control device of the invention, the target tire operating torque of the vehicle, which is proportional to the acceleration of the vehicle and not proportional to the target vehicle speed, is a control target for controlling the vehicle, and the target engine torque and the target engine speed are set on the basis of the target tire operating torque and the driving resistance. The term acceleration refers to acceleration or deceleration herein.
The target tire operating torque computing means easily sets the target tire operating torque such that the acceleration of the vehicle will not disturb the vehicle occupants. Therefore, the target tire operating torque computing means is a control target setting means.
If the target vehicle speed is set as a control target, as in the conventional driving control device, acceleration of the vehicle occurs when the target vehicle speed is changed. These parameters are not proportional. Therefore, to produce acceleration of the vehicle such that the vehicle occupants will feel no abnormality at a preset target vehicle speed, it is necessary to sample the optimum realizable acceleration of the vehicle and to precisely set, according to each vehicle driving condition, the operation characteristics of a target vehicle speed setting means on the basis of the sampling.
On the other hand, the acceleration xcex1 of the vehicle, the tire operating force Ftire [N] occurring at the vehicle tires, and the driving resistance Fload [N] (air resistance, tire rolling resistance, resistance due to the road gradient, and the like), which are applied to the vehicle during running, the mass of the vehicle M1 [kg], and an equivalent mass M2 [kg] for the inertia of rotating members of the vehicle, as shown in FIG. 1(a), can be expressed by the following equation (1) of vehicle motion.
(M1+M2)xc2x7xcex1=Ftire+Floadxe2x80x83xe2x80x83(1) 
From this equation, the acceleration xcex1 of the vehicle is proportional to the sum of the tire operating force Ftire, occurring at the vehicle tires, and the driving resistance Fload (the driving resistance Fload is of a negative value). In this invention, because the target tire operating torque, which is computed to direct the vehicle to a predetermined driving condition, is derived from the current driving conditions of the vehicle, the target tire operating torque includes a torque component corresponding to the vehicle driving resistance Fload and a torque component corresponding to the tire operating force Ftire. As a result, the target tire operating torque is proportional to the acceleration xcex1 of the vehicle.
Therefore, the target tire operating torque at which the vehicle acceleration is easily controlled without disturbing the vehicle occupants can be easily set by the target tire operating torque computing means using a single control law applicable to all driving conditions. This can be accomplished by setting, as a driving control target, the target tire operating torque of the vehicle, which is proportional to the acceleration of the vehicle. The target tire operating torque computing means can be tuned very easily as compared with the setting means in the conventional driving control device.
Furthermore, according to this invention, the target engine torque and the target engine speed are computed by the target engine control target computing means for producing the target tire operating torque. In the computation of these parameters, a control law that is set in accordance with the state of a lockup clutch is used.
According to this invention, therefore, each of these parameters can be optimized to account for the characteristics of the torque converter. Unlike the conventional control device, therefore, it is possible to control the engine optimally.
That is, in the conventional driving control device, an instantaneous value of the torque ratio is used to represent the characteristics of the torque converter when computing the target engine torque from the target vehicle speed, which is the control target. Therefore, when the engine speed has changed greatly or the torque converter slips significantly because the lockup clutch is disengaged, the target engine torque corresponding to the control target cannot be set. In the present invention, the control law, which has been set according to the state of the lockup clutch, is used in setting the target conditions (target engine torque and target engine speed) of the engine from the target operating torque, which is the control target. Thus it is possible to set the target engine torque and the target engine speed in accordance with the condition of the torque converter. Accordingly, it is also possible to optimally execute the engine control procedure with the engine control means.
The driving control device in one aspect of the invention sets the target engine torque and the target engine speed and then controls the engine accordingly to produce the target tire operating torque that has been set by the target tire operating torque computing means. In one form of the invention, the braking torque for decreasing the vehicle speed is applied only by the engine brake, which results in less control during deceleration of the vehicle.
When a high degree of control, not only during vehicle acceleration but also during vehicle deceleration, is required, the driving control device preferably has the features mentioned below.
In one form, the driving control device is designed to control a vehicle in which a braking device and the engine are controlled independently of the driver""s operation. In such a control device, like the driving control device mentioned earlier, the target tire operating torque computing means computes the target tire operating torque of the vehicle necessary to direct the vehicle to a specific driving condition on the basis of information received from sensors that detect the driving conditions of the vehicle. A driving resistance estimating means estimates the driving resistance of the vehicle on the basis of the information from the sensors.
Then, a controlled system selecting means selects either one or both of the engine and the braking device as a controlled system or systems on the basis of the tire operating torque and the driving resistance. When the braking device is selected, a target braking torque computing means computes the target braking torque for computing the target braking torque on the basis of the target tire operating torque and the driving resistance. A brake control means controls the braking device to achieve the computed target braking torque.
According to the control device of this form of the invention, the target tire operating torque set by the target tire operating torque computing means is a negative driving torque (in other words, a braking torque). When the target tire operating torque cannot be applied to the vehicle by the use of engine brake under the engine control, the braking device on the vehicle is driven to apply the target tire operating torque to reach the desired driving conditions.
In the control device of this form of the invention, when the engine is selected as the controlled system, the target engine torque and the target engine speed are computed, as described earlier, according to the state of the lockup clutch.
The target engine control target computing means computes the target engine torque and the target engine speed in accordance with the state of the lockup clutch (engaged or disengaged), which allows these parameters to be optimized according to the characteristics of the torque converter. That is, the target engine control target computing means changes the control law used in the computation of the target engine torque and the target engine speed according to whether the lockup clutch is disengaged or engaged. When the lockup clutch is engaged, the target turbine torque and the turbine speed of the torque converter are set as the target engine torque and the target engine speed.
On the other hand, when the lockup clutch is disengaged, a first target engine speed computing means computes the target engine speed on the basis of the target turbine torque and speed and the torque ratio and capacity factor of the torque converter. Then, the slope of the change of the target engine speed is computed by a target engine speed slope computing means. Subsequently, a first target engine torque is computed by a first target engine torque computing means on the basis of the turbine speed and the target engine speed. A second target engine torque is computed by the second target engine torque computing means on the basis of the target engine speed slope and the inertia of rotating members of the engine. A third target engine torque computing means serves to compute the target engine torque, which is the engine control target, on the basis of the first target engine torque and the second target engine torque.
Accordingly, the target engine condition (the target engine torque and the target engine speed) necessary to produce the target tire operating torque can be set in accordance with the characteristics of the entire power transmission system, including the dynamic characteristics of the torque converter, to optimize the control of the vehicle.
The control law of the target engine control target computing means is determined by the following procedure.
First, as shown by example in FIG. 1(b), the power transmission system includes an automatic transmission, a torque converter, which transmits the rotation of the engine to the input shaft of the automatic transmission, and a differential gear, which transmits the rotation of the output shaft of the automatic transmission to the right and left driving wheels.
In this invention, the target turbine torque computing means computes the target turbine torque Tt, which is the output torque of the torque converter, on the basis of the target tire operating torque, the driving resistance, and the gear ratio of the power transmission train (to be specific, the transmission ratio of the automatic transmission and the differential gears). The target engine control target computing means, therefore, is required only to set, in accordance with the power transmission characteristics of the torque converter, the engine torque Te and the engine speed Ne necessary to produce the target turbine torque Tt.
On the other hand, the power transmission characteristics of the torque converter differ between the engaged and disengaged states of the lockup clutch.
When the lockup clutch is disengaged, the torque converter transmits the rotation of the engine to the automatic transmission according to the power transmission characteristics of the torque converter. However, when the lockup clutch is engaged, that is, when the input shaft and the output shaft of the torque converter are coupled through the lockup clutch, the rotation of the engine is transmitted directly to the automatic transmission through the lockup clutch.
Therefore, it is understood that when the lockup clutch is engaged, the control law for setting the target engine torque and the target engine speed should set the target engine torque and the target engine speed such that the target engine torque Te equals the target turbine torque Tt and such that the target engine speed Ne equals the target engine speed Ne, which equals the turbine speed Nt.
However, when the lockup clutch is disengaged, the rotation of the engine is transmitted to the automatic transmission through the torque converter. Therefore, when setting the target engine torque and the target engine speed, it is necessary to take the power transmission characteristics of the torque converter into consideration.
When the torque converter is operating steadily at a nearly constant engine speed Ne, for example, during constant-speed driving, the engine torque Te can be expressed by the following equation (2), which refers to a capacity factor C(e) of the torque converter and the engine speed Ne. The turbine torque Tt is expressed by the following equation (3), which refers to the torque ratio tr(e) of the torque converter and the engine torque Te.
Te=C(e)xc2x7Ne2xe2x80x83xe2x80x83(2) 
Tt=tr(e)xc2x7Texe2x80x83xe2x80x83(3) 
where the capacity factor C(e) and torque ratio tr(e) of the torque converter are functions of the speed ratio Nt/Ne of the torque converter. Substituting equation (2) in equation (3), the turbine torque Tt is expressed by the following equation (4).
Tt=tr(e)xc2x7C(e)xc2x7Ne2 
Tt=tr(Nt/Ne)xc2x7C(Nt/Ne)xc2x7Ne2xe2x80x83xe2x80x83(4) 
Therefore, from equation (4), the target engine speed Ne for producing the target turbine torque Tr can be determined on the basis of the target turbine torque Tr and the turbine speed Nt (the first target engine speed computing means thus employs this relationship).
To set the target engine speed Ne from the target turbine torque Tt and the turbine speed Nt with the first target engine speed computing means, a two-dimensional map, which is used to set the target engine speed Ne from these two parameters Tt and Nt, is predetermined to apply to the torque converter. The target engine speed Ne may be determined by the use of this two-dimensional map. That is, it is possible to easily set the target engine speed by performing interpolation, in a well-known manner, using the two-dimensional map.
Next, the target turbine torque Te for producing the target turbine torque Tt is set according to equation (2). That is, the target engine torque Te can be set on the basis of the target engine speed Ne and the turbine speed Nt (the first target engine torque computing means employs this relationship).
Equation (2) expresses a relationship among the engine torque Te, the capacity factor C(e), and the engine speed Ne when the torque converter is operating steadily. However, when the engine speed Ne is transient, for example, during vehicle acceleration, a torque fluctuation component must be included in equation (2) due to the change of the engine speed Ne.
The torque fluctuation component can be expressed as a product of multiplication of the inertia Ie of the rotating members of engine and the differential (dNe/dt) of the engine speed Ne. Therefore, the target engine torque Te for realizing the target turbine torque Tt is given by the following equation (5).
Te=Ie(dNe/dt)+C(e)xc2x7Ne2xe2x80x83xe2x80x83(5) 
In one aspect of this invention, the slope of the target engine speed, which corresponds to the differential (dNe/dt) of the engine speed Ne, is computed by the target engine speed slope computing means. The second target engine torque computing means computes the second target engine torque, which corresponds to the engine torque fluctuation component, on the basis of the slope of the target engine speed and the inertia of the rotating members of the engine. Furthermore, the third target engine torque computing means determines the final target engine torque to be used to control the engine on the basis of the static target engine torque (the first target engine torque) given by the first target engine torque computing means and the dynamic target engine torque (the second target engine torque) given by the second target engine torque computing means.
The control laws for the computation of the target engine torque and the target engine speed by the target engine control target computing means have been explained. Some vehicles are equipped with a lockup clutch that not only can be engaged and disengaged, but also can be changed to a half-engaged state by a known lockup slippage control procedure. When the present invention is to be applied to such a vehicle, it is desirable that, in addition to the above-described two kinds of control laws, a control law should be set for computing the target engine torque and the target engine speed when the lockup clutch is in the half-engaged position.
To accomplish this, in another aspect of the invention, it is recommended that the target engine control target computing means be provided with a second target engine speed computing means for computing the target engine speed on the basis of the turbine speed and the amount of slippage of the lockup clutch when the lockup clutch is disengaged or is held in the half-engaged position under lockup slippage control.
The reason for the provision of the second target engine speed computing means will be explained below.
First, there are two kinds of lockup slippage control: an acceleration lockup slippage control and a deceleration lockup slippage control.
The acceleration lockup slippage control reduces the slippage of the torque converter by controlling the difference between the engine speed and the turbine speed to about 50 to 100 rpm and improves fuel economy by improving the efficiency of power transmission when the lockup clutch is half-engaged within a range in which the lockup clutch cannot be directly engaged.
The range in which the lockup clutch cannot directly be engaged is generally the low range of vehicle speed (e.g., 60 km/h or lower). Within this range, if the lockup clutch is directly engaged, unnatural sounds, transfer of engine vibration, longitudinal shaking of the vehicle on-off operation of accelerator, and the like may occur.
On the other hand, the deceleration lockup slippage control causes the lockup clutch to operate in a half-engaged position within the low speed range, so that the difference between the engine speed and the turbine speed will be about 50 to 100 rpm. As a result, the fuel economy will be improved.
In either of the acceleration lockup slippage control procedures, the engine speed Ne and the turbine speed Nt are computed to control the lockup clutch engaging force such that the difference between these speeds will be directed to a specific value (i.e., the amount of slippage, due to which Ne greater than Nt in acceleration lockup slippage control and Ne less than Nt in deceleration lockup slippage control).
The acceleration lockup slippage control procedure will be explained below by way of example.
When the acceleration lockup slippage control is normally executed, the difference between the engine speed Ne and the turbine speed Nt (Nexe2x88x92Nt) is controlled to the specific amount of slippage xcex94(xcex94=50 to 100 rpm).
Considering that, under the condition given above, the normal engine speed Ne is 1500 rpm or greater, the speed ratio of the torque converter (Nt/Ne) is given by the following equation (6), and the capacity factor of the torque converter at this time can be approximated as zero.
Nt/Ne=(1500xe2x88x92100)/1500=93%xe2x80x83xe2x80x83(6) 
Therefore, the torque to be transmitted from the engine to the automatic transmission is the transmission torque T1 of the lockup clutch, which will present no problem in practical use.
On the other hand, let Tin (Tin=C(e)xc2x7NE2) be the input torque of the torque converter as shown in FIG. 1(b), and the motions of the engine, torque converter, and lockup clutch in the slippage lockup conditions will be given by equations (7) and (8).
Te=Ie(dNe/dt)+Tl+Tinxe2x80x83xe2x80x83(7) 
Tt=tr(e)xc2x7Tin+Tlxe2x80x83xe2x80x83(8) 
Since the capacity factor C(e) can be approximated as zero, as described above, the input torque Tin will be zero in equations (7) and (8). Furthermore, in the steady state, the target engine speed slope (dNe/dt) in equation (7) also will become zero. Consequently, equations (7) and (8) will simplify to Te=Tl and Tt=Tl respectively.
Consequently, the engine torque Te, the transmission torque Tl of the lockup clutch, and the turbine torque Tt will become equal (Te=Tl=Tt).
Therefore, when the lockup clutch is engaged or is held in the half-engaged position by the lockup slippage control, the target turbine torque may be set as the target engine torque. Therefore it is unnecessary to provide a control law for setting the target engine torque.
On the other hand, the lockup slippage control controls the lockup clutch engaging force F1 to direct the difference between the engine speed Ne and the turbine speed Nt to the specific amount of slippage xcex94. Therefore, when the lockup clutch is controlled to be in the half-engaged state by the lockup slippage control, the target engine speed Ne can be calculated from the turbine speed Nt and the amount of slippage xcex94 of the lockup clutch.
To be more specific, when acceleration lockup slippage control is being executed, the target engine speed Ne is given by Ne=Nt +xcex94. When deceleration lockup slippage control is being executed, the target engine speed Ne can be given by Ne=Ntxe2x88x92xcex94.
In one aspect of the invention, when the lockup clutch is half-engaged position by the lockup slippage control procedure, the target engine speed is computed by the second target engine speed computing means on the basis of the turbine speed and the amount of slippage of the lockup clutch.
In one aspect of the invention, therefore, the target engine torque and the target engine speed can be optimized in accordance with the state of the torque converter. In this manner, it is possible to optimize the engine control of a vehicle that performs lockup slippage control.
The engine control means executes engine control on the basis of the target engine torque and the target engine speed, which have been determined by the target engine control target computing means. At least one of the target fuel injection quantity and the target throttle angle is computed as the controlled variable of the engine on the basis of the target engine torque and the target engine speed. It is recommended, therefore, that the engine be controlled in accordance with the computed controlled variable.
The driving control device of this invention is applicable not only to adaptive cruise control (ACC), which is for controlling a vehicle to follow a leading vehicle, but to controls other than ACC, such as vehicle stability control (hereafter simply VSC) for stabilizing a vehicle during a turn, braking slippage control (hereafter simply ABS (Anti-lock Brakes)) for limiting wheel slippage while braking, acceleration slippage control (so-called traction control, hereafter simply TRC) for limiting vehicle wheel slippage during acceleration, and constant-speed driving control (hereafter simply constant-speed CC (Cruise Control)) for constant-speed driving.
A plurality of driving control devices for executing such driving controls are often mounted on one vehicle. If, however, this invention is applied to every vehicle-mounted driving control device, it is conceivable that the target engine torque and the target engine speed, or the target brake torque, will be set by each control procedure if a plurality of driving control are executed at the same time, and the controlled variables for actual engine or brake control will not be optimized.
Furthermore, in such a case, there will be computing means for computing the target engine torque, the target engine speed, or the target brake torque by each driving control device. Therefore, there will be much waste, which adds to the cost of the vehicle.
Therefore, in one aspect of this invention, the target tire operating torque computing means is provided with a computing block for computing a target tire operating torque for accomplishing various different driving control procedures (the driving control procedures described above). From among a plurality of target tire operating torques inputted from various computing blocks, which correspond to the various control procedures, the target tire operating torque that has the highest priority under the current vehicle driving conditions is selected as the final target operating torque to be used for control according to predetermined conditions.
That is, according to the driving control device of this invention, a plurality of driving control procedures, such as ACC, VSC and ABS, can be accomplished by a single driving control device. The driving control device of this invention, therefore, simplifies the control system and reduces vehicle cost compared with a vehicle that has a plurality of driving control devices for respectively executing the various driving control procedures.
Furthermore, in the target tire operating torque computing means, the highest priority target tire operating torque is selected from the target tire operating torque determined by a plurality of computing blocks for the respective control procedures. It is therefore possible to efficiently set the control target of the engine, or the engine and braking device, and to employ the control target without a response delay.
Furthermore, another aspect of the invention is an integral control device for integral control of the vehicle operation. The computing block that computes the target tire operating torque for every driving control procedure can be independent of other computing blocks. It is therefore easier to design the control device, which lowers costs.
The target tire operating torque computing means selects the highest priority target tire operating torque. However, when the computing block that has the highest priority changes, the target tire operating torque will suddenly change. Therefore, in another aspect of the invention, when a new target tire operating torque is selected, the target tire operating torque computing means will preferably correct the newly selected target tire operating torque to create a smooth transition between the old target tire operating torque and the new target tire operating torque.
The target tire operating torque computing means is provided, as stated above, with a plurality of computing blocks for computing the target tire operating torque for various kinds of driving control procedures. Thus, in another aspect of the invention, one of the computing blocks is preferably for estimating the target tire operating torque needed by the driver on the basis of the position of the accelerator pedal and the vehicle speed. It is preferred that further computing blocks be for VSC, ABS, TRC, constant-speed CC, and for ACC, respectively.
That is, in this aspect of the invention, the target tire operating torque computing means includes a computing block, in addition to the computing blocks for performing various driving control procedures, for executing driving control based on the accelerator position. The driving control device is thus capable of optimally controlling the vehicle operation and of responding to the driver""s demands.
The target turbine torque is computed in the process for setting the final engine control variable from the target tire operating torque. In the computation of the target turbine torque, the driving resistance of the vehicle is used, in addition to the target tire operating torque and the gear ratio of the power transmission train, for the following reason.
The target tire operating torque is computed as the torque required for gaining a desired operation state assuming that the vehicle has a predetermined weight and is running on a flat road. However, the driving resistance Fload shown in FIG. 1(a) varies with changes in the road gradient such as upward and downward slopes and with changes in the weight of the vehicle. Therefore, in the computation of the target turbine torque, if the driving resistance Fload varies, it is necessary to correct the torque component of the target tire operating torque corresponding to the driving resistance Fload for the purpose of achieving the desired driving condition.
If such a correction is automatically performed by the first computing block to achieve the target tire operating torque on the basis of the driver""s operation of the accelerator, the vehicle will respond the same way to the position of the accelerator regardless of the road gradient and the vehicle weight. Generally, however, the behavior of a vehicle normally varies when the road gradient or the weight changes. Thus, the driver, when perceiving changes in the road gradient or vehicle weight, will change his or her driving. Therefore, automatic correction of the driving resistance in relation to the target tire operating torque that relates to the accelerator position is sometimes disturbing to the driver.
Thus, preferably, at least when the target tire operating torque based on the accelerator position has been selected as the final target tire operating torque by the target tire operating torque computing means, the target turbine torque computing means computes the target turbine torque on the basis of the selected target tire operating torque and the gear ratio of the power transmission train. Also, in this case, the driving resistance is preferably not used in the computation of the target turbine torque.
In another aspect of the invention, the target tire operating torque computing means is provided with a computing block for computing the target tire operating torque for ACC. Using the computing block, the target acceleration computing means computes the target acceleration of the vehicle necessary to follow the leading vehicle, on the basis of an input fed from the front recognition sensor, which detects the leading vehicle. The target acceleration is converted to the target tire operating torque by a conversion means, to set the target tire operating torque necessary to follow the leading vehicle.
Therefore, according to another aspect of the invention, the acceleration of the trailing vehicle can be optimally controlled, which allows the trailing vehicle to follow the leading vehicle, while keeping a proper distance between the two vehicles.
The correction of the target tire operating torque is computed on the basis of the target acceleration and the actual acceleration of the vehicle by the torque correction computing means, to correct the current target tire operating torque by the given correction and to set the final target tire operating torque.
The torque correction computing means is preferably constructed to evaluate at least a deviation between the target acceleration and the actual acceleration of the vehicle and an integral of this deviation. The torque correction computing means preferably sums values obtained by multiplying the deviation and the deviation integral by a proportionality constant and an integration constant (so-called proportional integral control action) to compute the torque correction.
In engine control and brake control, it is a general practice to set an upper limit (or a lower limit) for controlled variables of the engine and the braking device. Therefore, the engine control means, or both the engine control means and the brake control means, may include limiting means for performing this task.
However, if the limiting means is added to the engine control means (or the engine control means and the brake control means), the torque correction computing means will sometimes operate to increase the deviation integral by the proportional integral control action when the limiting means is functioning.
After the deviation integral is updated in a manner that increases the controlled variable, the controlled variable will remain greater than the upper limit for some period if the relationship of great and small between the target acceleration and the actual acceleration of the vehicle is reversed. In this state, the actual acceleration of the vehicle will continue, giving the vehicle occupants a feeling that something is abnormal.
A similar situation occurs when a device controlled by the control variable of the engine (or of the braking device) has reached a physical limit. That is, for example, when the engine brake is selected to decelerate the vehicle, an engine braking force is produced by, for example, closing the throttle valve. At this time, when the actual deceleration does not reach the target deceleration although the throttle valve is fully closed (i.e., when the throttle valve has reached a physical limit), the deviation integral will be updated in a manner that causes the controlled variable to exceed a value corresponding to the physical limit due to the proportional integral control procedure. Therefore, even when the target acceleration becomes positive, the throttle valve will be held fully closed for some period, which delays acceleration and gives the passengers a feeling of abnormality.
Therefore, in one aspect of the invention, when the torque correction computes the torque correction by the proportional integral control procedure, and when the engine control means (or the engine control means and brake control means) is provided with limiting means for setting a limit on the control variable or when the engine (or the braking device) has a physical limitation, it is preferred to provide the conversion means with a deviation integration prohibiting means for prohibiting the deviation integral value from being updated in a manner that would cause the control variable to surpass the set limit or to surpass a value corresponding to the physical limit.